The MGLAnnotationImage class is responsible for presenting point-based
annotations visually on a map view. Annotation image objects wrap UIImage
objects and may be recycled later and put into a reuse queue that is maintained
by the map view.

Declaration

The string that identifies that this annotation image is reusable. (read-only)

You specify the reuse identifier when you create the image object. You use this
type later to retrieve an annotation image object that was created previously
but which is currently unused because its annotation is not on screen.

If you define distinctly different types of annotations (with distinctly
different annotation images to go with them), you can differentiate between the
annotation types by specifying different reuse identifiers for each one.